Classifications

• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
  • G06F1/02—Digital function generators
  • G06F1/03—Digital function generators working, at least partly, by table look-up
  • G06F1/0321—Waveform generators, i.e. devices for generating periodical functions of time, e.g. direct digital synthesizers
  • G06F1/0328—Waveform generators, i.e. devices for generating periodical functions of time, e.g. direct digital synthesizers in which the phase increment is adjustable, e.g. by using an adder-accumulator
  • G06F1/0335—Waveform generators, i.e. devices for generating periodical functions of time, e.g. direct digital synthesizers in which the phase increment is adjustable, e.g. by using an adder-accumulator the phase increment itself being a composed function of two or more variables, e.g. frequency and phase
• • G—PHYSICS
  • G06—COMPUTING; CALCULATING OR COUNTING
  • G06F—ELECTRIC DIGITAL DATA PROCESSING
  • G06F2101/00—Indexing scheme relating to the type of digital function generated
  • G06F2101/08—Powers or roots

Definitions

• This invention relates to a method of digitally generating linear frequency modulated continuous wave (FMCW) waveforms at high frequency for use in such applications as high-frequency radar systems.
• FMCW linear frequency modulated continuous wave
• Signal sources used in HF radar systems require very high dynamic range, low phase noise and amplitude accuracy over a wide frequency band. In many applications they also require fast frequency switching and known phase characteristics. Typical signal sources use one of two methods to generate these signals - phase locked loop synthesis or direct digital synthesis.
• Phase locked loop methods in their most simple form, suffer from poor phase noise and/or poor frequency resolution (a relative large step between permissible frequencies). Frequency switching time can also be rather poor (long).
• Direct digital synthesis methods are usually based on phase accumulator techniques or memory look-up techniques. These techniques allow for both low phase noise and narrow frequency resolution.
• the phase accumulator design is optimised for the generation of a fixed frequency tone and the memory lookup technique suffers from large computational overheads as all data points must be calculated in advance.
• This invention combines and extends the phase accumulator technique to provide a device which can generate arbitrary or pseudo-arbitrary waveforms. This is particularly important in a real-time system.
• the invention described here may be used as a pseudo-ar f " trary waveform generator with high capability and low computational overhead.
• a common waveform used in HF radar systems is the linear frequency modulated continuous wave (FMCW) waveform, where the frequency is swept linearly up or down over a programmable frequency span in a programmable ' time interval, in order to generate these signals from conventional signal sources it is necessary to approximate the desired waveform by a series of short fixed-frequency steps.
• FMCW linear frequency modulated continuous wave
• This invention can produce true linear FMCW waveforms.
• the technique may be further extended to produce more complex waveforms of higher order, or these waveforms may be approximated by a piecewise linear approximation. This approximation, being a second order approximation, is inherently more accurate than the first order approximation of conventional phase accumulator techniques.
• the lookup ROM is not constrained to sinusoidal waveforms, but can be used to map any periodic function. In practice implementing further stages of accumulation becomes difficult due to a need for greater arithmetic precision in the early stages of accumulation, and to account for propagation delays through the accumulator chain.
• the design presented here approximates higher order waveforms by a piecewise linear approximation.
• an arbitrary waveform generator comprising : a plurality of accumulators each adapted to produce an output value from one or more input values; one or more memory means adapted to map the output of one or more accumulators to an amplitude value; a converter means adapted to convert the amplitude value from digital form to analogue form; and a control means adapted to synchronise the operation of the waveform generator.
• the converter means is a digital to analogue converter that converts a digital signal from the digital section of the generator to an analogue signal.
• the memory means is an addressable solid state memory device such as a read only memory device containing a look-up table for mapping a linear variation in phase to a sinusoidal variation in amplitude.
• the memory means could be an EPROM, Beta card, DRAM or other similar memory device.
• the memory device look-up table may contain other periodic functions.
• control means is a microprocessor incorporating a clock means which is a high purity oscillator.
• clock means which is a high purity oscillator.
• a filter means which filters the output of the digital to analogue converter. In practice this is a low pass filter.
• a method of direct digital synthesis of linear frequency modulated waveforms comprising the steps of : at a given regular time, adding a fixed frequency increment word to a frequency control value stored in a first register to produce a linearly increasing frequency control word; adding the frequency control word stored in the first register to a second register to form a quadratically increasing phase word; converting the quadratically increasing phase word to an amplitude value using a look-up table stored in a memory means to produce a linearly increasing frequency; and periodically resetting the frequency control word to produce a frequency sawtooth.
• the amplitude value is converted from a digital value to an analogue value using a digital to analogue converter.
• a filter means after the digital to analogue converter and in preference this is a low pass filter.
• a clock means to provide the given regular time and control the periodic resetting.
• FIG 1 is a schematic of an arbitrary waveform generator consisting of two phase accumulator stages.
• Phase accumulator signal synthesis is a digital technique whereby a fixed phase increment is added to a value stored in a phase register, giving rise to a linearly varying phase.
• the instantaneous frequency is defined to be the time derivative of the phase, the phase accumulator thus generates a fixed frequency signal.
• This signal is mapped to an amplitude by a sinusoidal lookup table, which may then be converted to an analogue form by a digital- to-analogue converter.
• the inputs to the generator are a frequency increment word 1 , an initial frequency 2 and an initial phase 3.
• the frequency register 4 is incremented in an adder 5 by the value of the increment word 1 on each reference clock pulse of a clock 6, giving a linear frequency progression.
• the linearly changing frequency output from the frequency register 4 is added in adder 7 to the phase register 8 on each clock pulse to produce a quadratic phase progression which is mapped by the ROM 9 to produce a linearly increasing frequency ramp.
• a digital to analogue converter 10 converts the digital signal 11 to analogue form which is subsequently passed through a low-pass filter T2 to produce the desired output.
• Each ramp can be completely defined by four parameters or control values : initial phase, initial frequency, frequency increment word and duration of ramp. Any of the first three parameters may be unused (taking the final value of the previous ramp), which allows for greater flexibility in waveform generation and reduces some computational overhead.
• a logical extension of this technique is to implement more stages of accumulation to generate polynomials of higher order, allowing even more complex waveforms to be generated directly.
• current technology imposes restrictions on the capability of such higher-order polynomials, such that it is presently more appropriate to generate these higher-order polynomials in a piecewise linear approximation using short linear FMCW ramps.
• the method of controlling this dual phase accumulator allows independent setting of the initial phase, initial frequency, and frequency deviation rate. It also allows pseudo-arbitrary waveforms to be generated relatively simply by means of piecewise linear approximation with short time intervals (possibly as short as 10-20 microseconds). This is a very powerful method of generating pseudo-arbitrary waveforms, as the length of the waveform sequence is dependent only on the storage requirements for the waveform definition, rather than on the storage requirements for the entire sequence (as in the memory lookup method of arbitrary waveform synthesis). It also allows real ⁇ time generation of data points, avoiding the long overheads of memory lookup techniques.
• the waveform generator may be configured to either repetitively generate the same ramp or produce a series of independent ramps.
• Pseudo-arbitrary waveforms can be generated by a piecewise linear approximation of ramps to the desired waveform instead of using a multiple accumulator architecture of higher order.
• each ramp segment is defined by four parameters only it is possible to reduce the minimum ramp duration to the time required to transfer these four parameters to the appropriate registers.
• a minimum step size of 10 to 20 microseconds is a physically achievable value that will provide a good approximation to most desired waveforms.
• the pseudo-arbitrary waveform is implemented as a series of short frequency ramps approximating the desired waveform.
• the total number of ramp segments that can be put in a sequence has yet to be determined, but will number in the thousands and will be limited only by parameter storage requirements.
• the speed of programming and implementing these ramps as well as the maximum number of ramps is determined only by the speed and storage capabilities of the controlling microprocessor. This is a very powerful method of producing pseudo-arbitrary waveforms and allows very complex waveforms of long duration to be generated relatively simply without recourse to multiple accumulator architectures.
• the invention offers a number of advantages.
• the output frequency can be changed very rapidly without impacting on the quality of the output signal.
• the non-pipelined nature of the design allows the output frequency to change within a single sampling clock period.
• any changes in frequency are controlled to provide non-discontinuous changes in phase and frequency unless discontinuity is desired, in which case the discontinuity is known and can thus be controlled.
• the dual accumulator is optimised for quadratic phase generation (i.e. linear FMCW).
• quadratic phase generation i.e. linear FMCW.
• the addition of further stages of phase accumulation provide a method for optimised generation of higher order waveforms. Being all digital the phase and amplitude are controlled at all times. This has particular importance in radar systems where a coherent detection process is implemented. Coherent detection requires a known, repeatable phase progression and phase errors translate directly to errors in detection.

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• Engineering & Computer Science (AREA)
• Theoretical Computer Science (AREA)
• Physics & Mathematics (AREA)
• General Engineering & Computer Science (AREA)
• General Physics & Mathematics (AREA)
• Stabilization Of Oscillater, Synchronisation, Frequency Synthesizers (AREA)
• Radar Systems Or Details Thereof (AREA)

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• 1992
  • 1992-06-23 CA CA 2112252 patent/CA2112252A1/fr not_active Abandoned
  • 1992-06-23 JP JP5501199A patent/JPH07502151A/ja active Pending
  • 1992-06-23 WO PCT/AU1992/000305 patent/WO1993000737A1/fr not_active Application Discontinuation
  • 1992-06-23 EP EP93901022A patent/EP0591477A4/fr not_active Withdrawn

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